Extending PET chains with pyromellitic anhydride and other crosslinking agents is known, for giving PET the necessary consistency for receiving the blowing agent, thus obtaining an expanded and stable foam. As a result, during the following extrusion phases of the foam, more or less significant phenomena of polymer degradation may arise, which in any case represent problems with respect to the quality of the final product.
Preparing a thermoplastic polymer foam in the presence of epoxides, crosslinked by means of photosensitive catalysts is also known (U.S. Pat. No. 6,323,251), wherein the crosslinking reaction of the epoxide is effected downstream of the injection of the blowing agent, in particular outside the extrusion. The known process is consequently not suitable for providing a foam having the necessary consistency for the formation of the final product. In addition, photosensitive catalysts only act on the surface layer of the foam, which does not allow the crosslinking reaction to also take place inside the foamy mass.